About Vivian Richmond

Can someone let me know what id fan fiction? I have heard so much about it and I tried to do a research about it but the internet is full of so many things that it makes learning so complicated sometimes. I have no idea what is fan fiction or who are fan fiction novel writers. Of how little I perceived from the internet study it is related to some real-life thing and fans make an alternative of it, or something like that. It is getting really confusing. It will be so helpful if someone can demonstrate this concept with clarity.